EMI absorbing shielding for a printed circuit board

ABSTRACT

The present invention provides printed circuit boards, EMI shields, and methods that provide an EMI absorbing material and an EMI reflecting material. In one embodiment, the EMI shield comprises a shield body and an EMI absorbing material coupled to the shield body.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a division of U.S. application Ser. No.10/871,874, filed Jun. 18, 2004, which claims benefit of U.S.Provisional Patent Application No. 60/479,460, filed Jun. 19, 2003, thecomplete disclosures of which are incorporated herein by reference intheir entirety for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates generally to EMI shielding. Morespecifically, the present invention relates to EMI shielding thatcomprises an absorbing material that is capable of absorbingelectromagnetic radiation.

All electronic products emit electromagnetic radiation, generally in therange of 50 MHz. to 1 GHz., but not limited to this range especially inlight of the many advances in high-speed microprocessor design and therapidly increasing capabilities of high-speed networking and switching.The problem of emittance of electromagnetic radiation is not new todesigners of electronic equipment; indeed, significant efforts are takento reduce electromagnetic interference (EMI) and electromagneticradiation (EMR) and virtually every country has a regulating agency (FCCin the U.S., for instance) that controls the marketing and sale ofelectronic equipment that do not pass stringent requirements forEMI/EMR, whether radiation or intercepted (also called susceptibility)by an electronic device.

Present day solutions typically involve the use of conductively paintedplastic housings, conductive gaskets, and/or metal cans that are affixedto a printed circuit board by soldering or similar methods, some ofwhich are semi-permanent. In virtually all cases, the existing solutionsare expensive and add to the cost of manufacturing electronic equipmentsuch as cell phones, personal digital assistants, laptop computers,set-top boxes, cable modems, networking equipment including switches,bridges, and cross-connects.

More recently, technology for the metallization of polymer substrateshas been in evidence. For example, Koskenmaki (U.S. Pat. No. 5,028,490)provides a polymer substrate that is layered with aluminum fibers andsintered to form a flat material with a metal layer that is intended toprovide EMI control (also a legal requirement of the FCC and otherforeign entities and generally referred to as electromagnetic complianceor EMC). This product was manufactured and sold by the 3M Corporation.As of approximately 2002, this product was withdrawn from the market.The material was shown to be expensive, difficult to use, and subject toinferior performance due to cracking of the metalized layer.

Gabower (U.S. Pat. No. 5,811,050) has provided an alternative approachwherein a thermoformable substrate (any number of different kinds ofpolymers) is first formed, and then metalized. This approach offers theadvantage of not subjecting the metalized layer to the stresses createdduring molding. The product has been shown to be highly effective andrelatively low-cost.

The major methods of providing for a conductive coating or layer on asubstrate generally include (1) selective electroless copper/nickelplating, (2) electroless plating, (3) conductive paints and inks, and(4) vacuum metallization. Collectively, these are referred to herein as“metallization methods.” In each of these typical applications, either aplanar or formed substrate of metal or plastic is “treated” to form aconductive shield. The ultimate quality, performance, and cost for eachmethod varies widely but ultimate a metalized thermoformable shield isformed into an (1) integral solution that surrounds the printed circuitboard in some manner (a.k.a., “enclosure” level solution), or (2) formedinto a compartmentalized shield that fits on the guard traces of the PCB(“board” level solution), or (3) formed into smaller shields that fitover individual components using the guard traces (“component” levelsolution).

When it comes to EMI shielding at the board of component level, thefeature deployed today is to place a conductive surface of the EMIshielding in contact with the guard traces either (1) directly bymetalizing a shield surface and placing it in contact with the trace or(2) by metalizing the “outside” surface (from the perspective of thecomponent being shielded) and then using some method of attachment thatconnects the guard trace with the metalized outside surface. The purposeof the guard traces, based upon the historical use of soldered metalcans, is to provide a point of contact between the EMI shielding and thePCB that can be subject to standardized surface mount technology (SMT)solder reflow processes that ultimately provide a solid connectionbetween the metal can shield and the PCB. And, while the metal can andguard trace become grounded at least one point to the signal, power, orground plane(s), the amount of peripheral contact between the shield andmetal can is largely for the purpose of achieving a tight mechanicalseam.

The resultant assembly of the EMI shielding and the electronic componentprovides adequate shielding for many applications; however, as thefrequency of chips increase and the data transmission rates increase,the creation of errant radiation (EMI) becomes much easier and moreharmful to circuits and components located nearby. Indeed, with theincreasing density of chips, the subject of immunity (of one chiprelative to another) becomes all the more important. Thus, in general,conventional EMI solutions will increasingly find themselves inadequatefor purposes of immunity and indeed, radiated emissions may also becomean increasing issue. Furthermore, for microwave devices, especiallythose that operate of have harmonic frequencies above about 10 GHz.radiated emissions will be a significant concern.

The radiation fields within an inner space defined by a printed circuitboard and an EMI shield are comprised of very complex combinations ofboth electric fields (E-fields) and magnetic fields (H-fields) that arebouncing off chip and shield structures forming very complex fields withmany resonances. These resonances can be very strong in terms of fieldstrength and can easily be observed at frequencies that are troublesomefrom an EMC perspective. In general, there is nothing to contain theradiation escaping from the bottom of the chip except for the phenomenaof reflection from the ground plane (the “image” plane) which can, insome situation, improve the radiation emissions problem but isproblematical from a design and manufacturing point to achieve.

While electromagnetic radiation (EMR) fields are very complex, thebehavior of EMI shielding can be determined from a measurement ofshielding effectiveness (SE). Typically, this is done in the far fieldwhere the EMR fields are distinctly plane wave in form. In the nearfield, EMR is either reflected or absorbed and for the most part, withthe drive for lightweight devices and shielding, reflection has been theonly viable method of shielding. Controlling EMR is then a matter ofdesigning either solid or intermittent shielding around the chip thatincreases the SE.

A phenomenon that may limit the effectiveness of EMI shieldingstructures is often referred to as cavity or enclosure resonances-thetendency of enclosed spaces with reflective surfaces to develop standingwaves of varying amplitudes. It is not unusual for the peaks of thestanding waves to exceed limits for radiated emissions. The wavelengthof these resonances in a simplified sense is a function of a halfwavelength, the dimensions of the structure and its configuration(square, rectangle, circle, etc.) with a constraint involving wholenumbers of half wavelengths. In real situations, these cavity resonancesare detuned (shifted in frequency and reduced in amplitude) from theoryby the presence of intermediate or internal structures (i.e., chips andelectronic components). In general, only sophisticated numericalanalysis codes can predict these resonances. More importantly, there arenot any readily acceptable methods to altering the frequency of theresonances or their amplitudes.

The presence of these difficult-to-treat resonances makes EMC designdifficult. Therefore, what are needed are methods and EMI shielding thatcan absorb resonances within the EMI shielding.

BRIEF SUMMARY OF THE INVENTION

The present invention provides EMI shielding, shielded circuit boards,shielded electronic devices and methods of shielding a printed circuitboard.

In one aspect, the present invention provides an EMI shield. The EMIshield comprises a shield body and an EMI absorbing material coupled tothe shield body. The EMI shield provides both EMI reflective and EMIabsorbing properties.

The shield body may be comprised of one or more layers of an insulativesubstrate, such as a thermoformable resin or polymer substrate. Aconductive layer, such as a metal layer, may optionally be coupled to orotherwise applied to the polymer substrate. In alternative embodiments,the insulative substrate may be impregnated or otherwise treated to makethe substrate conductive and no additional conductive layer is needed.

The conductive layer will be grounded and may have a thickness andresistivity that is sufficient to substantially reflect EMI. Theconductive layer typically has an average thickness between about 1.0micron and about 50.0 microns. In some configurations, the conductivelayer will have a surface resistivity between about 0.01 ohms per squareand about 3 ohms per square. In other embodiments, the conductive layermay have a higher surface resistivity, if desired.

In one embodiment, the shield body comprises a first layer and the EMIabsorbing material is coupled to the first layer. The first layertypically includes the polymer substrate and the conductive layer, butit may only include a polymer substrate (conductive or non-conductive).In other embodiments, the shield body comprises a first layer and asecond layer and the EMI absorbing material is positioned in between thefirst layer and the second layer. In yet other embodiments, a protectivepolymer layer may be applied over the conductive layer to physicallyseparate the conductive layer and the EMI absorbing layer. In suchembodiments, the conductive layer and the EMI absorbing layer may stillbe electrically coupled to each other, if desired.

The ability of the EMI absorbing material to absorb electromagneticenergy is based in part on the permittivity and permeability of thematerials used in both the conductive layer, substrate and EMI absorbingmaterial. Furthermore, conductive materials with a higher resistivity,which can dissipate electromagnetic energy into heat, have proven to beeffective in absorbing electromagnetic energy. Materials such as theMu-metal, stainless steel, nickel and other fabricated alloys are knownto have the ability to absorb magnetic fields. Other materials whileless appropriate for absorbing magnetic energy, nevertheless have theability to reflect and alter the physical location of energy within aconfined space. Applicants have found that the EMI absorbing materialsof the present invention are able to absorb electric fields and magneticfields between about 100 Khz. and about 3 GHz., or more.

In some embodiments, the EMI absorbing material may take the form of ahighly resistive conductive layer (e.g., steel, aluminum, copper, etc.),a high permittivity and permeability such as Mu-metal, a carbon felt,and at least partially conductive or metalized reticulated foamstructure, or the like.

In embodiments that use a high resistivity conductive layer to dissipatethe H-fields, EMI absorbing material will have a higher resistivity thanthe conductive layer. The volume resistivity of the high resistivityconductive layer is typically between about 50 ohms and about 500 ohms.Alternatively, it may have a surface resistivity of between about 50ohms per square and about 500 ohms per square. In other embodiments, theEMI absorbing materials may have a higher or lower resistivity. Forexample, in the embodiments which rely on a high permittivity and a highpermeability (which is higher than the conductive layer), such EMIabsorbing materials do not have to require such high resistivity levels.

In one embodiment, the EMI absorbing material has an open-celledskeletal structure that is at least partially conductive. The opencelled skeletal structure typically has a conductive layer (e.g., metallayer) over at least a portion of the open-celled skeletal structure. Ascan be appreciated, instead of a metal layer over the open-celledskeletal structure, the open-celled skeletal structure itself may becomposed of a conductive material. The EMI absorbing material may beconductive throughout, conductive along only some surfaces, orconductive only along a portion of the material.

In preferred embodiments, the EMI absorbing material comprises anopen-celled skeletal structure that is composed of reticulated foam. Thereticulated foam may have a variety of different pores per inch, but istypically between about 10 pores per inch and about 80 pores per inch.The reticulated foam may have differing pores per inch along its lengthor the same pores per inch along its length. The reticulated foammicrostructure itself may be conductive (e.g., it may be carbon loaded)or the reticulated foam may be metalized along at least one surface. Themetal layer on the reticulated foam typically has an average thicknessthat is less than about 1 micron. Specifically, the metal layer may bebetween about 0.5 micron and about 0.8 micron, and more preferablybetween about several angstroms to less than 0.1 micron.

The EMI absorbing material may fill only a portion of a chamber definedby the shield body, or it may completely fill the chamber. In someembodiments, the EMI absorbing material is in contact with theelectronic component within the chamber, and in other embodiments, theEMI absorbing material is positioned within the chamber to leave openspaces around the electronic component.

The EMI absorbing material may be grounded in a variety of ways. Forexample, the EMI absorbing material may be directly contacted with aground trace, in electrical contact with one or more grounded conductivelayers of the shield body, and the like.

In another aspect, the present invention provides a shielded printedcircuit board. The printed circuit board comprises a substratecomprising a ground plane. An electronic component is coupled to thesubstrate. An EMI shield is positioned on the substrate over theelectronic component and is electrically coupled to the ground plane.The EMI shield comprises a shield body and an EMI absorbing materialcoupled to the shield body. The EMI absorbing material comprises bothEMI reflective and EMI absorbing properties.

The EMI shield is typically coupled to the ground plane through asurface ground trace. The printed circuit board may further comprise anetwork of vias that are in electrical communication with both the EMIshield and the ground plane. The network of vias provides an EMI shieldinterior of the printed circuit board.

A further understanding of the nature and advantages of the inventionwill become apparent by reference to the remaining portions of thespecification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a conventional method of shielding an electroniccomponent on a printed circuit board.

FIG. 1A illustrates one exemplary EMI shield encompassed by the presentinvention that includes a polymer substrate and one or more conductivelayers.

FIG. 1B is a top view of a flange of an EMI shield that has selectivelyspaced holes that may receive a conductive or non-conductive adhesive.

FIGS. 1C and 1D illustrate a ball of adhesive and a flattened ball ofadhesive, respectively, that is positioned on a flange, and throughstrategically placed holes in the flange of an EMI shield of the presentinvention.

FIG. 2 illustrates a printed circuit board and an EMI shield thatcomprises an EMI absorbing material that completely fills an interiorspace of the EMI shield.

FIG. 3 illustrates a printed circuit board and an EMI shield in which aportion of the EMI absorbing material is conductively coated and aportion of the EMI absorbing material is non-conductive.

FIG. 4 illustrates a printed circuit board and an EMI shield in whichthe EMI absorbing material only fills a portion of the interior space ofthe EMI shield and does not contact the electrical component.

FIG. 5 illustrates an embodiment of an printed circuit board and an EMIshield which includes an EMI absorbing material positioned between afirst substrate layer and a second substrate layer.

FIG. 5A illustrates a conductively filled aperture on an EMI shield thatgrounds an EMI absorbing material to a ground trace.

FIG. 5B illustrates a direct contact between the EMI absorbing materialwith a surface ground trace.

FIGS. 6 and 7 illustrate embodiments of a printed circuit board and anEMI shield that contacts a surface of the electronic component.

FIG. 8 illustrates an EMI shield that comprises a polymer layer betweena conductive layer and an EMI absorbing material.

FIG. 9 illustrates a polymer substrate positioned between the conductivelayer and the EMI absorbing material.

FIGS. 10 and 11 illustrate simplified methods of manufacturing variousembodiments of the EMI shield of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides printed circuit boards and EMI shieldsthat comprise an integration of a shield body (e.g., a shaped resin orpolymer substrate such as a metalized thermoform) with an EMI absorbingmaterial to provide an EMI shield that is effective in terms of bothreflection and absorption of electric and magnetic fields emitted froman electronic component to mitigate the effects of chamber resonances.

The EMI shields of the present invention may comprise an EMI absorbingmaterial. The EMI absorbing material may completely fill a space definedby the shield body, cover all inner and/or outer surfaces of the shieldbody, or cover only selected portions of the inner and/or outer surfacesof the shield body. Further, the EMI absorbing material may be locallyplaced for particular applications and still provide important EMIabsorption.

EMI absorbing material typically has a high permittivity andpermeability (i.e., good ability to store electric and magnetic energy,respectively) and/or a high surface resistivity so as to dissipate andabsorb magnetic fields within the EMI shield. The EMI absorbing materialincludes, but is not limited to, a magnetic or conductive metal,Mu-metal, carbon felt, an at least partially conductive or metalizedreticulated foam structure, or the like.

A number of natural metals (e.g., iron, nickel, etc.) are known to haveproperties which allow them to absorb energy in the form of magneticfields and these same materials have the ability to conduct electricfields. The degree to which any material offers both reflective (in thesense of E-field management) and absorptive (in terms of H-field ormagnetic field management) depends upon the both the permittivity andpermeability of the material. Indeed, through proper layering of thinfilms of various metals, it is entirely possible to construct specialtylayers that provide a good balance of electronic and magneticproperties. A number of man-made fabricated metals (stainless steel,Mu-metal, etc.) have higher intrinsic permeability thus being useful asthin layers for the absorption of magnetic fields.

In one configuration, the EMI absorbing material is a reticulated foamthat is composed of a lightweight polymeric material that has anopen-celled skeletal structure. The open-celled structures may becomprised of a wide variety of polymers and the size and shape of theskeletal structure is dictated by the particular process chosen. Thereticulated foam may be provided with a variety of different pores perinch, but is typically between 10 PPI and about 80 PPI.

The reticulated foam may or may not have one or more conductive or metallayers deposited on at least a portion of the open-celled skeletalstructure. The reticulated foam can provide absorptive qualities eitherby being made of a metal, a conductive polymer material, or a polymercomposite material (one in which the polymer is loaded with additionalmetallic particles or flakes, such as carbon). In embodiments whichinclude a conductive layer on the reticulated foam, depending on thethickness and resistive property of the conductive layer, the conductivelayer can enhance either the reflective, absorptive, or any combinationthereof of the underlying foam structure.

In one embodiment, one or more conductive layers are applied to thereticulated foam. The conductive layer typically comprises aluminum,copper, nickel, silver, tin, steel, gold or the like. The conductivelayers on the foam usually have a thickness less than about 1 micron,and preferably between about 0.5 micron and about 0.8 micron, and morepreferably between about several angstroms to less than 0.1 micron, onthe structure throughout the desired portion of the open-celled skeletalstructure. In some, but not all embodiments, the conductive layer on thereticulated foam has a surface resistance between about 50 ohms persquare and about 500 ohms per square.

The open-celled skeletal structure may be metalized along one or moresurfaces. For example, the open-celled skeletal structure may bemetalized throughout the structure so as to maintain electricalcontinuity through the open celled skeletal structure. In otherembodiments, the open celled skeletal structure may be metalized alongall interior and exterior surfaces except one exterior surface that isintended to contact a printed circuit board, traces, or an electroniccomponent. In some configurations, the open celled skeletal structuremay be comprised of a conductive material or the polymer skeletalstructure may include conductive particles or fibers integrally formedin the polymeric microstructure and thus may not need a metal layer.

Reticulated foam can be easily cut into various shapes and formedthrough various types of molding processes to form to the interiorshapes corresponding to the electronic components and/or shield body.The reticulated foam is typically easily compressed so that, in general,the foam can be oversized and then compressed to fit into an interiorspace defined by the shield body and the enclosed components. When thereticulated foam is metalized in some fashion (e.g., vacuum deposition,paint, electroless plating, electroless plating, or electroplating, forinstance), the reticulated foam becomes an EMI shielding material thathas both reflective properties (by virtue of the conductive materialselected for coating) and absorbing properties (by virtue of thecharacter of the polymer as in, for instance, polymer, a conductivepolymer or a polymer that contains conductive particles (graphite,nickel coated particles, etc.)). A more complete description of a methodof metalizing a reticulated foam is described in commonly-owned andco-pending U.S. patent application Ser. No. 10/230,966, entitled “EMIAir Filter,” filed Aug. 28, 2002, the complete disclosure of which isincorporated herein by reference.

The EMI absorbing materials of the present invention is not limited toreticulated foam. The reticulated foam may be replaced with any similaropen cell structure (either regular or irregular in geometric structureand form) and comprised of a combination of material constructions suchas conductive polymer, conductive particle loaded polymer, or metal.Alternatively, other useful materials for absorbing magnetic fields or“H fields” are comprised of a carbon felt, a thin, resistive metal layer(e.g., aluminum, copper, tin. Mu-metal, etc.) typically having athickness less than about 1 micron, or any resistive, conductivesubstance such as a conductive polymer (polyaniline, for instance) thatcan dissipate magnetic fields into heat.

FIG. 1 illustrates a shielding solution in which a PCB 10 with anemitting electronic component 12, such as a semiconductor chip, and EMIshield 14 are depicted (not to scale). The EMI shield 14 is placed onsurface ground traces 16 on a first surface of the PCB through solderingor other connector assemblies that provides for electrical continuityand enclosure of the electronic component 12. Radiation 18, shown asradiating waves that, may establish standing waves within the EMI shield14 and also “leak” out of the printed circuit board 10 through theunderlying nonconductive elements of the printed circuit boardstructure, e.g., the PCB glass/polymer structure, as for instance, flameretardant four-layer board, e.g., FR4. In FIG. 1, radiation 18 is shownas bouncing off an internal ground plane 20 and emerging either into theenvironment or adjacent to another chip (not shown). It should beappreciated that the radiation fields are comprised of very complexcombinations of both electric and magnetic fields that are bouncing offelectronic components and shields forming very complex fields with manyresonances. These resonances can be very strong in terms of fieldstrength and can easily be observed at frequencies that are troublesomefrom an EMC perspective.

FIG. 1A illustrates a simplified cross-sectional view of an EMI shield14 that is encompassed by the present invention. EMI shield 14 maycomprise one or more compartments that receive and shield the electroniccomponent 12. In embodiments in which the EMI shield 14 has multiplecompartments for multiple electronic components, each of thecompartments will be sized and shaped to separate the electroniccomponents from each other. As such, the EMI shield 14 may take on avariety of shapes, sizes and forms so as to conform to the specificshape and configuration of the printed circuit board and electroniccomponents being shielded. The EMI shields 14 of the present inventiontypically include a shield body 22. The shield body 22 may be comprisedof one or more layers such as a non-conductive polymer film (e.g., athermoformable film that comprises polybutylene terephthalate, polyvinylchloride, polycarbonate, polyethylene terephthalate, polyethyleneterephthalate with Glycol, a Polycarbonate/ABS blend, or the like,extruded films that have been “loaded” with conductive particles orEMI/RFI absorbing materials (e.g., RAM or radar absorbing materials,such as used in stealth aircraft), conductive polymers, metal films, orthe like.

In preferred embodiments, the shield body 22 is comprised of a polymersubstrate 30 that can be formed by a variety of plastic processingmethods to a desired shape to partially or fully enclose electroniccomponent 12 on printed circuit board 10. In exemplary embodiments, thepolymer substrate 30 is a thermoformable polymer that is shaped usingthermoforming techniques (e.g., vacuum, pressure, or mechanical forces).The polymer substrate 30, however, may be shaped using any conventionalor proprietary methods.

The shield body 22 of the EMI shield 14 typically has at least oneconductive or metal layer 32 on at least one side of the polymersubstrate 30. Metal layer(s) 32 will have one or more layers and has athickness that is sufficient to block or reflect the transmission ofEMI, typically between about 1 micron and about 50 microns. The metallayer may comprise one of more layers of aluminum, nickel, copper, orthe like.

The metal layers 32 of the present invention may optionally be appliedto the polymer substrate 30 after shaping of the resin film layer. Ifthe metal layer 32 is applied prior to shaping of the polymer substrate30, the shaping process (e.g., thermoforming) tends to stretch out andweaken portions of the metal layer 32. Such stretching and thinning hasbeen found to weaken and sometimes destroy the EMI shieldingcapabilities of the metal layer 32. The EMI shields 14 of the presentinvention will generally have a substantially even thickness in themetal layer that is sufficient to block the passage of EMI. A moredetailed description of some embodiments of an EMI shield that may beused with the present invention is described in commonly owned U.S. Pat.Nos. 5,811,050 and 6,768,654, and commonly owned U.S. patent applicationSer. No. 09/788,263, entitled “EMI & RFI Shielding for Printed CircuitBoards,” filed Feb. 16, 2001, U.S. patent application Ser. No.09/685,969, entitled “EMI & RFI Containment Enclosure for ElectronicDevices,” filed Oct. 10, 2000, and PCT Patent Application No. 00/27610,entitled “EMI Containment Apparatus,” filed Oct. 6, 2000, the completedisclosures of which are incorporated herein by reference.

Typically, the metal film layer 32 is deposited onto one or moresurfaces of the polymer substrate 30 using vacuum metallization. Whilethe illustrated embodiment shows a single metal layer on an innersurface of polymer substrate 30, it should be appreciated one or moremetal layers may be applied to at least one of the inner surface and theouter surface of the polymer substrate 30. Vacuum metallization is onepreferred method because of the substantially even layer of metal thatcan be applied to the shaped polymer substrate 30 to create the EMIshield 14. It should be appreciated however, that other methods ofdepositing the metal layer 32 to the polymer substrate 30 could be usedwithout departing from the scope of the present invention. For example,instead of vacuum metallization, other methods such as a depositing arandom mat or fiber weave, sputtering, painting, electroplating,deposition coating, electroless plating, laminated conductive layers,and the like, may be used to deposit the metal layer onto the shapedresin film layer. The metal layer 32 will typically be grounded to aground plane 20 with a surface ground trace 16 and/or at least some ofadditional vias so as to create a Faraday cage around the electroniccomponent 12.

In the simplified embodiment of EMI shield 14 shown in FIG. 1A, the EMIshield comprises a top surface 34 and a plurality of sidewalls 36 thatextend at an angle from the top surface so as to define a chamber orinner space 26. A flange 38 may extend laterally from the plurality ofside walls and extends in a plane that is substantially parallel withthe first external surface 28 of printed circuit board 10. In preferredembodiments, the top surface, side walls and flange are metalized on atleast one surface.

Metal cans EMI shields may be coupled to surface ground trace 16 using asolder reflow process. However, since the polymer melt temperature ofthe polymer substrate 30 is usually lower than the reflow temperatures,the reflow process is generally not applicable to resin based EMIshields. As such it is desirable to use non-solder connectors to couplethe EMI shield 14 to the ground plane 20 in the printed circuit board14. The connectors include, but are not limited to a conductiveadhesive, removable or fixed mechanical connectors, rib and grooves, andthe like. A more complete description of suitable connectors for usewith the EMI shields of the present invention are described in commonlyowned, co-pending U.S. patent application Ser. No. 10/789,176, entitled“Methods and Devices for Connecting and Grounding an EMI Shield to aPrinted Circuit Board,” filed Feb. 26, 2004, the complete disclosure ofwhich is incorporated by reference.

As shown in FIG. 1B, if the metalized thermoform EMI shield 14 containsa flange 38, openings 40 may optionally be selectively placed on theflange 38 where either conductive or nonconductive adhesive 42 orsimilar conductive material (even solder) could be placed over theopenings 40 to couple a conductive portion of the flange 38 (and the EMIabsorbing material) to the surface ground trace 16 and/or vias. Such aconfiguration is particularly beneficial when a metal layer 32 is placedon the outside surface of EMI shield 14 so that the conductive adhesivecreates an electrical path to the metal layer on the outside surface ofthe EMI shield. The type of adhesive and its properties may be chosen toallow also for easy removal if repair of the underlying electroniccircuits or components are required. One suitable adhesive is the 3M®PSA adhesive (3M part numbers 9713 and 9703).

While not illustrated, it should be appreciated that instead ofselective placement of adhesive into opening 40, if desired, asubstantially continuous line of adhesive may be placed onto the flange(e.g., between the flange and the printed circuit board or onto a uppersurface of the flange) to mechanically and/or electrically coupled theflange to the surface ground trace 16 or other grounding elements (e.g.,vias).

Referring now to FIGS. 1C and 1D, adhesive balls 44 may be placed eitherbefore or after the EMI shield 14 is placed onto printed circuit board10. Thus, the adhesive balls 44 may be positioned on top of the flange38 or between the flange 38 and printed circuit board 10. Since EMIshield 14 does not have to have a total continuous peripheral electricalcontact with the vias and/or ground trace 16, either conductive ornonconductive adhesive may be used. If desired, as shown in FIG. 1D, theadhesive balls 44 may be flattened to reduce the profile of the adhesiveball. Additional methods for coupling an EMI shield to a printed circuitboard can be found in commonly owned and co-pending U.S. patentapplication Ser. Nos. 10/789,176, entitled “Methods and Devices forConnecting and Grounding an EMI Shield to a Printed Circuit Board,”filed Feb. 26, 2004 and Ser. No. 10/825,999, entitled “ElectromagneticInterference Shielding for a Printed Circuit Board,” filed Apr. 15, 2004the complete disclosures of which are incorporated herein by reference

While the preferred embodiments of the present invention include avacuum metalized thermoform shield body 22, the present invention is notlimited to such EMI shields 14. For example, instead of metalizedthermoforms, the present invention may use metal cans, polymer basedshield with a fiber mat, injection molded plastic enclosures, andfabricated sheet metal assemblies, or the like. For ease of reference,the EMI shields 14 illustrated in FIGS. 2-7 will be shown in asimplified form as a shield body 22. It should be appreciated, that theshield body 22 may encompass any of the EMI shields 14 described herein,including a vacuum metalized thermoform EMI shield body 22 thatcomprises one or more metal layers 32, as illustrated in FIG. 1A.

FIG. 2 shows an embodiment of an EMI shield 14 and printed circuit board10 that are encompassed by the present invention. In the illustratedembodiment the EMI shield 14 comprises a shield body 22 that comprisesat least one metal layer (not shown) and an EMI absorbing material 24coupled to an inner surface 25 of the shield body 22. The inner surfaceof the shield body 22 may be the polymer substrate or it may be the oneor more metal layers. In the illustrated embodiment EMI absorbingmaterial 24 may be configured to completely fill the interior space 26defined by the printed circuit board 10, electronic component 12 and theinner surface 25 of the shield body. In such embodiments, the surface ofthe printed circuit board 28 and the electronic component 12 disposedwithin the space 26 will be coated with an insulative layer so as toprevent shorting or interference with any leads or conductive elementsexposed on the first surface of the printed circuit board.

While not shown, in most embodiments, a metal layer 32 of EMI shield 14will be in electrical contact with the EMI absorbing material 24. Forexample, metal layer 32 may be disposed along an inner surface of theshield body 22 and in direct contact with the EMI absorbing material 24.As shown in FIG. 2, the metal layer (not shown) on flange 38 will alsobe in contact with the surface ground trace 16 so that the EMI absorbingmaterial 24 may be electrically grounded (See also FIG. 1A). In otherembodiments, the EMI shield 14 may not comprise metal layer 32, and theEMI absorbing material 24 may directly contact a grounding element, suchas ground trace 16 (not shown). While FIG. 2 illustrates one method ofgrounding the EMI shield 14, a variety of other methods of grounding theshield body 22 and the EMI absorbing material 24 are available and arewithin the scope of the present invention including the application of aconductive adhesive at selected contact points, mechanical fasteners,and special features built into the containing form designed tospecifically hold the interior absorbing materials.

FIG. 3 illustrates an embodiment of the present invention in which theEMI absorbing material 24 within the inner space 26 is not conductivethroughout its thickness. In the illustrated embodiment, a first portion50 of the EMI absorbing material 24 is conductive (e.g., metalized) anda second portion 52 of the EMI absorbing material 24 is non-conductive.The first conductive portion 50 will typically be in contact with aconductive surface (e.g., metal layer) of the EMI shield 14 so as to begrounded. The non-conductive portion 52 may contact the traces andelectronic component 12 on the printed circuit board without worry ofinterfering with the signals from the electronic component.

As can be appreciated, the first portion 50 and second portion 52 may becomposed of a single piece of EMI absorbing material or it may becomposed of a plurality of different pieces attached to each other. Inone configuration, EMI absorbing material 24 is formed of a partiallymetalized reticulated foam. If the reticulated foam is thick enough andhas high number of pores per inch (PPI) and only one side of the foam isvacuum metalized, the metal would not metallize all the way through thefoam. Table I illustrates a thickness of the reticulated foam versus itspores per inch and illustrates whether or not vacuum metallization of asingle side would result in a fully (“Thru”) or only partially metalized(“Partial”) foam.

TABLE I Thickness of reticulated foam versus penetration PenetrationThickness/PPI 10 PPI 18 PPI 28 PPI 38 PPI 50 PPI 80 PPI 0.1 inch ThruThru Thru Thru Thru Partial 0.2 inch Thru Thru Thru Thru Partial Partial0.3 inch Thru Thru Thru Partial Partial Partial 0.5 inch Thru PartialPartial Partial Partial Partial 1.0 inch Partial Partial Partial PartialPartial Partial

While the embodiment of FIG. 3 would typically fall into a category of“partial” metalized reticulated foam, it may be possible to couple afully or partially metalized or conductive reticulated 24 to anon-metalized reticulated foam to produce the EMI absorbing material 24illustrated in FIG. 3. In such embodiments, the first portion 50 mayhave the same or different PPI as the second portion 52. The firstportion and second portion may be coupled using any convention method,such as an adhesive, or the like. As can be appreciated in otherembodiments, two or more different portions of the reticulated foam maybe coupled to each other to provide different PPI portions. The densityof the absorbing structures (as reflected in the parameter of PPI orpores per inch) has a strong influence on the relative amounts ofreflection and absorption. Thus, the higher the PPI, the more dense thereticulated foam and there will be improved absorption and reflectanceof EMI. Conversely, the lower the PPI, the larger the pores and a higheramount of frequency is allowed to pass through, and there will be lowerreflection and absorption

FIG. 4 illustrates another embodiment of an EMI shield 14 encompassed bythe present invention in which the interior space 26 defined by the EMIshield 14 is not filled with the EMI absorbing material and the EMIabsorbing material 24 does not contact the printed circuit board or theelectronic component. In such embodiments, an open space 54 surroundsthe electronic component 12. The EMI absorbing material 24 willtypically be in contact with an inner surface 25 of the EMI shield 14and make a grounding connection to a grounding element, such as groundtrace 16, through a conductive portion of the EMI shield (e.g., a metallayer).

FIG. 5 shows another configuration encompassed by the present inventionin which the shield body of the EMI shield 14 is comprised of aplurality of layers 58, 60. The EMI absorbing material 24 may be placedbetween first layer 58 and second layer 60. In embodiments in which thefirst layer and second layer are polymer resin layers, the first layer58 and second layer 60 may be metalized on one or both of their surfaces(not shown). If one or both of the polymer substrate layers 58, 60 aremetalized, then the one or more metal layers on the first and secondlayers 58, 60 may be electrically coupled to a ground plane 20 throughelectrical contact with a surface ground trace 16. For example, as shownin FIG. 5, a metal layer (not shown) on an inside surface of first layer58 may be directly contacted with the ground trace 16. In theillustrated embodiment, the metal layer of first layer 58 is inelectrical contact with EMI absorbing material 24, which in turn mayalso be in electrical contact with an optional metal layer of secondlayer 60.

Alternatively, one or both of first and second layers 58, 60 may benonconductive (e.g., not metalized). If the first and second layers arenot metalized, the EMI absorbing material 24 may be connected to theground trace 16 on the printed circuit board 10 through direct contactof the EMI absorbing material 24 to a ground trace 16, or through holes32 placed in the flanges of the EMI shielding 10 through whichconductive adhesive or similar conducting coupling member is placed (Seefor example FIGS. 1B to 1D).

For example, in the embodiment of FIG. 5A, both the first layer 58 andthe second layer 60 may comprise flanges 38 and the foam extends betweenthe flanges 38 of the first layer 58 and second layer 60. The firstlayer 58 and second layer 60 may comprise a conductive layer on at leastone of the surfaces. An aperture 100 may be created in the via and aconductive element 102, such as a conductive adhesive or solder may fillthe via and create a conductive path between the ground trace 16 and theEMI absorbing material (and the conductive layer on the first or secondlayers 58, 60).

In yet another embodiment shown in FIG. 5B, the EMI absorbing materialmay be positioned to directly contact the ground trace. A mechanicalconnector may be used to couple the EMI shield 14 so that the EMIabsorbing material 24 is positioned over the ground trace 16.

A more complete description of useful methods of coupling and groundingan EMI shield to a printed circuit board is described in co-pending andcommonly owned U.S. patent application Ser. No. 10/789,176, filed Feb.24, 2004, entitled “Methods and Devices for Connecting and Grounding anEMI Shield to a Printed Circuit Board” and U.S. patent application Ser.No. 10/825,999, filed Apr. 15, 2004, and entitled “ElectromagneticInterference Shielding for a Printed Circuit Board,” the completedisclosures of which are incorporated herein by reference.

In some embodiments, the EMI shield 14 and/or reticulated foam 24 may bedesigned to interface with the electronic component 12, as shown inFIGS. 6 and 7. The illustrated configuration of FIGS. 6 and 7 modify andreduce the internal geometry of the internal space 26 and changes theresonances, which may prove useful in certain situations especially whenharmonics are in evidence. These harmonics are difficult to controlexcept either through geometrical modification of the containing cavityor via EMI absorbing materials. Furthermore, the electronic component 12may optionally be modified to contain internal metallic structures orsurfaces 62 that rise to the top surface of the electronic component 12where contact is made with the EMI shield 14, thus further enhancing theoverall shielding effectiveness of the EMI shield 14.

As shown in FIGS. 6 and 7, a conductive surface 61 of the EMI shield 14may touch off onto a conductive surface 62 of electronic component 12.If the contacted surface 62 of the electronic component 12 is tied to aground plane 20, then the contact with the conductive surface of the EMIshield provides another grounding point for the EMI shield 14. If themetal surface 61 of the EMI shield is not otherwise tied directly to aground plane 20 (e.g., through a surface ground trace), then theelectrical contact between the conductive surface 61 of the EMI shield14 and the surface 62 of the electronic component 12 provides a moredirect return path for EMI noise to the ground plane 20.

Similar to the embodiment illustrated in FIG. 5, the EMI absorbingmaterial of FIG. 6 is positioned between a first layer 58 and a secondlayer 60 of the EMI shield 14. Because the absorbing material ispositioned between the first layer 58 and second layer 60, any EMI noisethat passes from the electronic component 12 into the EMI absorbingmaterial may be re-reflected between the first layer 58 and second layer60 through EMI absorbing material 24. For example, as illustrated by thethree arrows in EMI absorbing material 24, EMI noise may pass from theelectronic component 12 through the second layer 60 of the EMI shieldand either be absorbed and returned to ground by the EMI absorbingmaterial 24 or be reflected by an metal layer formed on the first layer58 and reflected back into the EMI absorbing material 24 until the EMIis completely dissipated and/or returned to ground.

In any situation, a certain amount of energy is reflected leaving theremaining energy to pass through the conductive layer. The permeabilityand permittivity of the conductive layer dictates how much is reflectedand how much is absorbed. Energy passing through the EMI absorbingmaterial 24 will tend to physically excite the EMI absorbing material 24thereby dissipating a portion of its energy by converting it to thermalenergy or heat. The remaining energy reaching the far side of the metalconductive material can exit, albeit with much less energy or it may bereflected back through the EMI absorbing material 24 thereby dissipatingenergy further. Varying the conductivity of the EMI absorbing material24 affects the materials absorption characteristics. Most EMI absorbingmaterials are semi-conductive in nature. Many are carbon filled orgraphite filled to promote some conductivity across the materialssurface or through the volume of the material. If the absorbing materialis too conductive, it may reflect more EMI noise than it absorbs,thereby negating or minimizing EMI absorption through thermalconversion. Having a minimally conductive material help improve the EMIabsorption capabilities of the material by providing electricallyexcitable particulates, such as the graphite or carbon particulates,which improve the thermal conversion of EMI noise. Similarly, by lightlymetalizing (e.g., a metal layer over the open-celled skeletal structurehaving an average thickness between about 0.01 micron and about 0.5micron), the foam structure becomes semi-conductive in nature andthereby performs in a similar fashion to the carbon or graphite filedmaterials. While it should be appreciated that the exact conductivitylevel or the reticulated foam material could be varied through ourmetallization techniques to absorb a specific EMI noise frequency range,a common volume resistivity level for reticulated foam absorbingmaterial would be between 50 ohms and 500 ohms.

In another embodiment of the present invention shown in FIG. 7, theshield body may be comprised of only a single layer 58 and the two ormore interior spaces 26, 26′ defined by the substrate 58 maybe partiallyor completely filled with the EMI absorbing material 24. While FIG. 7shows an open space 54 underneath the EMI absorbing material 24, any ofthe EMI absorbing configurations of FIGS. 2 and 3 may be provided, inwhich the entire interior space is filled. In such embodiments, the EMIabsorbing material 24 and the shield body 22 are grounded throughelectrical connection to surface ground traces 16 and/or surface 62. Insome applications which involve the use of nonconductive (plastic orceramic) packaging, the EMI absorbing material 24 may be in contact withthe IC and circuits. In other applications, it may be desirable to havethe EMI shield 14 be comprised of a nonconducting layer contiguous withthe electronic component 12. The nature of the invention is such thatconductive and nonconductive surface treatments can be deployed withrelative ease.

FIG. 8 illustrates yet another embodiment encompassed by the presentinvention. Similar to the previous embodiments, the EMI shield 14 shownin FIG. 8 comprises a shield body 22 that is composed of a polymersubstrate 30 and a conductive layer 32. As shown in FIG. 8, theconductive layer 32 is on an inner surface of the polymer substrate andmay be directly attached to the surface ground trace 16 that ispositioned beneath the flange 38. Unlike the previous embodiments, apolymer layer 63 may be applied over the metal layer. The polymer layer63 will have thickness that is less than the thickness of the polymersubstrate. Typically, the polymer substrate 30 and the polymer layer 63will be composed of different materials. For example, the polymersubstrate 30 is typically a thermoform that comprises polybutyleneterephthalate, polyvinyl chloride, polycarbonate, polyethyleneterephthalate, polyethylene terephthalate with Glycol, aPolycarbonate/ABS blend, or the like, while the polymer layer 63 istypically sprayed onto the conductive layer 32 and is composed ofpolyurethane, polyethethylene, polyethethylene terephthalate,polypropylene, polyvinyl chloride, polycarbonate, polybutyleneterephthalate, or the like. Polymer layer should be of sufficientthickness to prevent unwanted electrical coupling between conductivelayer 32 and EMI absorbing material 24 which may diminish the absorbingproperties of EMI absorbing material 24.

An EMI absorbing material 24 may be coupled to an inner surface of thepolymer layer 63 so as to position the EMI absorbing material within thechamber 26 defined by the EMI shield 14. Similar to the otherembodiments, the EMI absorbing material 24 will have at least one of ahigher resistivity and a higher permeability and permittivity than theconductive layer 32 so that the EMI absorbing material 24 will haveproperties that provide for H-field absorption.

To allow for electrical coupling between the EMI absorbing material 24and the conductive layer 32, during application of the polymer layer 63to the conductive layer 32, a portion of the metal layer may be masked,so as to provide for an electrical path between the conductive layer 32and the EMI absorbing material.

In some instances it may be desired to have the EMI absorbing material24 coupled to the polymer substrate 30 instead of the conductive layer.Thus, while not shown, the position of the absorbing material 24 and theconductive layer 32 as shown in FIG. 8 may be switched.

FIG. 9 illustrates yet another embodiment encompassed by the presentinvention. In the embodiment illustrated in FIG. 9, the polymersubstrate may have a first, inner surface 65 and a second, outer surface67. In the illustrated embodiment, the EMI reflecting conductive layer30 is coupled to the inner surface 65 of the polymer substrate 30 andthe EMI absorbing material 24 is coupled to the outer surface 67 of thepolymer substrate. In other embodiments (not shown), the EMI absorbingmaterial is coupled to the inner surface 65 and the EMI reflectingconductive surface 32 is coupled to the outer surface 67.

Similar to the other embodiments of the present invention, the EMIabsorbing material 24 will have at least one of a higher resistivity anda higher permeability and permittivity than the conductive layer 32 sothat the EMI absorbing material 24 will absorb H-fields.

To allow for grounding of the EMI absorbing layer and the conductivelayer 32, a variety of connector members may be used. For example, inone embodiment, an opening (FIG. 1B) may be created through the flange38 of the EMI shield 14, and a conductive element 42 may be used tomechanically and electrically couple the EMI absorbing material 24 andconductive layer 32 to the surface ground trace 16.

FIGS. 10 and 11 illustrate simplified methods of manufacturing variousembodiments of the present invention.

Referring again to FIGS. 2-9, to improve electrical shielding within thesubstrate of the printed circuit board 10 a plurality of conductiveelements, typically in the form of conductively coated or filled vias64, may optionally be selectively formed in the layer(s) of the printedcircuit board 10 so that at least some of vias 64 extend from a groundedlayer, such as a ground plane 20 to first surface 28 of printed circuitboard 10. As can be appreciated, not all of the vias 64 in printedcircuit board 10 needs to extend to the surface 28 of the printedcircuit board 10. Moreover, some of vias 64 may be used to interconnectone grounded layer to another grounded layer, such as the ground plane20.

Typically, vias 64 extend substantially orthogonal from a plane of thesurface 28 of the printed circuit board 10 to the ground plane 20 andare formed using conventional methods. Vias 64 may be created in thelayers of printed circuit board 10 so that one end of the via extends tothe external surface 28 to provide a topside surface to which anelectrical connection to the EMI shield 14 is possible. At least aportion of the conductive via 64 may be in contact with ground plane 20.Consequently, when an EMI shield is conductively contacted with via 64on the first external surface 28, the EMI shield 14 will be grounded.

The network of vias 64 usually provides between as few as about fourvias and as many as several hundred vias that extend from the surface 28down to ground plane 20 around each electronic component 12. Typically,the vias 64 will be formed in a shape that corresponds to the shape ofthe perimeter of the flange 38 or sidewall 36 of the EMI shield 14, soas to provide a via-shield grounding contact along the perimeter of EMIshield 14. Thus, the shape of the network of via 64 and ground planewill depend on the shape of the corresponding EMI shield (e.g., if theshield perimeter is round, the vias will be positioned in a circlearound the electronic component; if the shield perimeter is rectangular,then the vias 64 will be positioned in an independent rectangle aroundthe electronic component).

The number of vias 64 utilized may be determined by the operatingfrequency of the electronic component or any harmonic frequenciesthereof. In the case of higher operating frequencies, too few vias wouldpotentially allow radiation to leak through in between the vias 64. Athigher frequencies, wavelengths of the radiation are shorter and areable to leak out in between smaller spaces. Therefore, if there are toofew vias 64, then the vias will be spaced further apart from one anotherand would allow for more radiation to leak through.

The number and locations of the vias 64 may be based according to theoperating frequencies of the electronic components within the electronicdevice that is being shielded. Preferably, the vias 64 are placed adistance apart from one another that is approximately equal to betweenabout ½ and about ¼ of the wavelength of the highest frequency orharmonic thereof to create an effective shield and prevent radiationfrom leaking out from between the vias 64. For example, the adjacentvias 64 may be spaced apart between about 1 mm and about 200 mm,depending on the wavelength of the highest frequency.

Typically, via 64 are plated with copper, nickel, gold, silver, tin orsolder (typically tin/lead combo) and the like. Vias 64 are generallyplated through an electroless or an electrolytic plating process. Theplating can extend through the vias 64 and be exposed on the flatsurfaces of the printed circuit board which would allow a small ringletof the conductive surface of the via 64 to be exposed and allowed tomake contact with the EMI shield or ground trace.

The diameters of vias 64 can range between 0.015″ and 0.040″ in somecases. The smaller the diameter of the via 64, the more expensive ittypically is to manufacture the printed circuit board. In addition, ifthe via 64 diameter is too small, it would be difficult to conductivelyplate the entire depth of the via. On the other hand, if the diameter ofthe via 64 is too large, when solder is applied to the printed circuitboard it may well up and create a bump of solder on the board, which canbe undesirable. Also, if the via diameter is too big, when thenon-conductive solder mask is applied, it could drape into the via 64,thereby creating a depression in the printed circuit board which mayalso be undesirable.

The number of the vias 64 positioned along each side of an EMI shieldwould depend on the operating frequency of the components beingshielded. The higher the frequency, the closer the vias 64 would beplaced together and therefore the more vias would be placed along eachside of the shield. The height of the vias 64 is dependant on the numberof layers on the printed circuit board and how many layers the via wouldneed to pass through to reach the ground plane 20. For instance, afour-layer printed circuit board is typically 0.064″ thick total(˜0.016″ per layer). Vias 64 could pass between one layer or between allfour layers. This same would hold true for printed circuit board withhigher numbers of layers.

As shown in FIGS. 2-7, the plurality of vias 64 form an interconnectednetwork of spaced conductive elements that extend throughout the innerstructure of printed circuit board 10 to form an open, mesh-like EMIshield for the electronic component 12. When connected with an externalEMI shield 14, the combination provides an EMI shield that substantiallywholly surrounds a volume of the printed circuit board beneathelectronic component 12 and reduces the emittance of electromagneticradiation to surrounding electronic components. In both embodiments, thetop of the EMI shielding is substantially solid (though EMI shield 14may contain ventilation holes). While the bottom portion (e.g., theconductive vias and grounded layer) is not “solid,” but is more of amesh or cage and the spacing between the vias are small enough tosubstantially reduce the amount of electromagnetic interference thatwould escape. A more complete description of the vias may be found incommonly owned, co-pending U.S. patent application Ser. No. 10/825,999,entitled “Electromagnetic Interference Shielding for a Printed CircuitBoard” filed on Apr. 15, 2004, the complete disclosure of which isincorporated herein by reference.

While the preferred embodiments of the present invention have beendescribed, a person of ordinary skill in the art will recognize thatvarious modifications, substitutions and equivalents may be used withoutdeparting from the spirit and scope of the present invention.

1. An EMI shield comprising: a shield body comprising a conductive layercoupled to a polymer substrate; a polymer layer coupled to theconductive layer; an EMI absorbing material coupled to the polymer layerwherein the EMI absorbing material comprises an open-celled skeletalstructure and the open-celled skeletal structure comprises a reticulatedfoam; and wherein the reticulated foam comprises a metal layer on allinterior surfaces and on all exterior surfaces except a lower surfacethat is configured to be in contact with at least one of a printedcircuit board, an electronic component, or traces on the printed circuitboard.
 2. The EMI shield of claim 1 wherein the EMI absorbing materialis electrically coupled to the conductive layer through an opening inthe polymer layer.
 3. The EMI shield of claim 1 wherein the open-celledskeletal structure is conductive.
 4. The EMI shield of claim 1 whereinthe shield body comprises a top surface and a plurality of sidewallsthat extend at an angle from the sidewalls so as to define a chamberthat receives an electronic component.
 5. The EMI shield of claim 4wherein the EMI absorbing material only partially fills the chamberdefined by the shield body.
 6. The EMI shield of claim 4 wherein the EMIabsorbing material substantially fully fills the chamber defined by theshield body.
 7. The EMI shield of claim 1 wherein the reticulated foamcomprises a metal layer throughout the open-celled skeletal structure soas to maintain electrical continuity throughout the open-celled skeletalstructure.
 8. The EMI shield of claim 1 wherein the reticulated foamcomprises between about 10 pores per inch and about 80 pores per inch.9. The EMI shield of claim 1 wherein the metal layer comprises anaverage thickness of less than about 1.0 micron over the open-celledskeletal structure.
 10. The EMI shield of claim 9 wherein the metallayer comprises a volume resistivity of between about 50 ohms and about500 ohms.
 11. The EMI shield of claim 10 wherein the metal layer on theopen-celled skeletal structure comprises a higher resistivity than theconductive layer.
 12. The EMI shield of claim 1 wherein the EMIabsorbing material comprises a higher permittivity and a higherpermeability than the conductive layer.
 13. The EMI shield of claim 1wherein the EMI absorbing material comprises a Mu-metal.
 14. The EMIshield of claim 1 wherein the EMI absorbing material comprises a carbonfelt.
 15. The EMI shield of claim 1 wherein the polymer layer comprisespolyurethane, polyethethylene, polyethethylene terephthalate,polypropylene, polyvinyl chloride, polycarbonate, or polybutyleneterephthalate.
 16. The EMI shield of claim 1 wherein the polymer layeris thinner than the polymer substrate.